The reduction of harmful emissions from engines or other thermal engines plays a significant role in fulfilling environmental protection efforts, which are becoming larger and larger. This relates to emissions originating directly from the combustion process, on the one hand, but also emissions which originate from secondary procedures in or on the engine, on the other hand. In this case, this can relate to externally active emissions, on the other hand, but also procedures inside the engine can be included in this case, for example, the fuel introduction into the lubricant oil or the recirculation of blow-by gases into the combustion chamber. To be able to reduce the emissions, it is primarily necessary to detect and evaluate the actually occurring emissions. In this case, in particular lubricant oil emissions and unburned hydrocarbons are significant. These have to be able to be measured rapidly with a high level of accuracy, to also be able to depict internal engine procedures with sufficient dynamic response.
Various measurement systems are known in the prior art. The unburned hydrocarbons in the exhaust gas are determined with a high level of chronological resolution by flame ionization detectors. This method can claim the advantage that it is less complex. However, it is not specific by its nature, i.e., a determination of a mass spectrum of the detected molecules cannot be performed. Therefore, this method is excessively coarse and does not fulfill the modern demands with respect to an accurate assignment of the hydrocarbons according to fuel and oil fractions.
Mass spectrometers are used for accurate characterization of hydrocarbons. They consist of an ion source, a mass analyzer, and a detector.
The chronological resolution of the system is substantially also determined by the mass analyzer.
In a known embodiment, this mass analyzer is embodied as an electrical quadrupole, to which a voltage source is connected, so that a periodically oscillating electrical field arises (U.S. Pat. No. 2,939,952). Because of the field, only ions having determined, specific mass/charge ratio run on stable paths, all others are unstable and are eliminated. A time-consuming sequential measurement of the individual masses (scanning) is necessary to generate a mass spectrum. The realistic measurement time for generating a spectrum of 50 to 550 atomic mass units is 500 ms.
In time-of-flight mass spectrometers (TOF-MS) (DE 10 2012 203 150 A1), different ionic species of a sample are accelerated in an electrical field. Subsequently, the ions pass through a flight route. The different velocities of the various ionic species have the result that the ions having different mass-charge ratio separate with respect to the movement direction. At the end of the mass analyzer, the ions fall on the ion detector, which measures the frequency of the successive ions simultaneously. A time-of-flight spectrum in the range of 50 to 550 atomic mass units can be recorded in less than 20 μs. To achieve better mass accuracy and detection limit, a mass spectrum is calculated in 1 ms from multiple time-of-flight spectra.
If a double-focusing sector field mass spectrometer in Mattauch-Herzog geometry is used (DE 10 2010 056 152 A1), the energy bandwidth of the ion beam is reduced in the electrostatic analyzer, to achieve a high resolution of the mass separation in the following magnetic field. All ionic masses can be depicted simultaneously in one focal plane due to the geometry. A planar detector enables the simultaneous detection of the complete mass spectrum. A time-consuming sequential measurement is not necessary.
A typical technology for ionization of molecules in mass spectrometry is electron impact ionization (EI) at 70 eV. Depending on the high ionization energy in this hard ionization method, fragmentation of the molecules into smaller fractions occurs, which cannot be unambiguously assigned to the substances in a mixture.
The inadequacies of this technology have resulted in the development of soft ionization methods, in which essentially molecular ions are generated. Different technologies based on chemical ionization (CI), field ionization (FI), and photoionization (PI) have been developed. The use of matrix-assisted laser ionization (MALDI) and electrospray ionization (ESI) is widespread for polar molecules.
In the case of photoionization, molecular ions can be generated by targeted selection of the photon energy. The use of UV radiation results in a high level of selectivity in the case of aromatic hydrocarbons and is generated, for example, by pulsed lasers (REMPI; laser-based resonance enhanced multi-photon ionization). The detection of organic materials can be performed by single photon ionization (SPI) using VUV radiation (vacuum ultraviolet).
A further soft ionization method is based on taking samples using supersonics (SMB, supersonic molecular beam) and subsequent ionization of the energetically cold molecules using electron impact ionization (cold EI), which is described in U.S. Pat. No. 6,617,771 B2.
The required combination of detection limit, discrimination power, selectivity, and measurement speed of the known systems do not correspond to the demands currently placed on the observation of hydrocarbon emissions in dynamic engine procedures.
An improved method for determining the lubricant oil content in the exhaust gas is known from WO 2005/066605 A2. According to this, the exhaust gas mixture taken as a sample is supplied to an ion source and, after ionization, supplied to a combination comprising a mass spectrometry filter unit, which is designed as a multipole, and a detector unit.
The filter unit is embodied so that a specific transmission range is defined for mass-charge numbers to be transmitted. A lubricant oil fraction to be measured is therefore defined. The measurement over this fraction is carried out using the mass spectrometer as a global measurement of the intensity in one step simultaneously over the entire transmission range. This measurement system enables outstandingly rapid measurement with a measurement time of 1 ms over a settable measurement range. The dynamic response of this measurement system is good, but the spectral resolution is not completely satisfactory.